1. Field of the Invention
The present invention relates generally to heat exchangers and methods of controlling heat exchangers.
2. Discussion of the Background
Boilers are utilized as heat exchangers in numerous applications. Boilers typically utilize heat transfer liquid within a tube to absorb heat from an outside source, and then transfer a substantially liquid-free vapor fraction of the heated fluid to a desired location for use. The heated fluid within the boiler is typically in both a gaseous phase and a liquid phase. Boilers are particularly advantageous for use in a configuration in which it is desired to prevent the presence of heat transfer fluid in a liquid phase at a location downstream of the boiler.
A related art boiler 430 is depicted in FIGS. 4A and 4B. The boiler 430 is provided along a flue 414. The boiler 430 includes a first manifold 432, a second manifold 442 provided at an elevation directly vertically above the first manifold 432, and heat exchanger tubes 452, 454, 456, and 458 fluidly connecting the first manifold 432 to the second manifold 442. A first fluid contacts an outer surface of the tubes, and a second fluid is provided within an interior of the tubes.
FIG. 4A depicts the boiler 430 in a non-operational state, and FIG. 4B depicts the boiler in an operational state. In the non-operational state depicted in FIG. 4A, the liquid phase level in each of the conduits 452, 454, 456, and 458 are at an identical vertical height and parallel to the horizon due to gravitational forces acting on the second fluid and uniform temperature distribution of the second fluid in the conduits. In the operational state when the first fluid flow C is present as depicted in FIG. 4B, the conduits located upstream in the first fluid flow B will be in contact with the highest temperature first flow. As the conduits absorb heat from the first fluid flow A, then each succeeding downstream conduit will be in contact with first flow at a sequentially lower temperature. This temperature distribution will shift the non-operational state of the liquid phase level (depicted in FIG. 4A) to the liquid phase levels depicted in FIG. 4B.
The inventors have noted that the above shift in liquid phase levels in the conduits tends to create a situation in which the liquid phase level of the downstream conduit 458 wants to rise to a level at which it reaches the second manifold 442. If the liquid phase level reaches the second manifold 442, it may contaminate system components of the second fluid flow located downstream of the boiler 430. Additionally, if the liquid phase level reaches the second manifold 442, the second fluid in the liquid phase may cascade into the other conduits and create a circular flow (counterclockwise in FIG. 4B) of liquid phase fluid within the boiler, which would significantly degrade the efficiency of the boiler. Since the water cascading from the tube 458 is at or below the saturation temperature at the operating temperature, such cascading overflow will cool the tubes 452, 454 and 456, potentially quenching them to a temperature at or below saturation. This quenching causes a temporary interruption of vapor production, which can cause serious damage to downstream systems supplied by the boiler.
Commonly-used boilers ensure a constant flow of substantially dry vapor by separating the vapor in one or more centrifugal, or cyclone, separators, followed by metering through a control valve. This requires maintenance of a predetermined water level, usually at a point in the manifold 442 as well as the operation of one or more high temperature vapor metering valves. The liquid level must also be measured in such a scheme, usually by use of a commonly-used level sensor selected from the family including sight glasses, mechanical floats, ultrasonic, radar and capacitance. Such sensors are usually confronted by one or more problems such as a severe sensitivity to overheating, susceptibility to corrosion and fouling, large size, high cost and complexity, and low resolution. Because many non-contact sensors such as radar also require large minimum distances for sensing, they are poorly suited to operation in small boilers.
Prevention of tube overflow problems using traditional level sensors requires an increase the size of the system and tends to provide a system that is not robust against operational upsets.
It is therefore desirable to provide a boiler system that overcomes the above restrictions.